This invention relates to a conductive composition to be located between the skin and an electrode element when a biological electrode is applied to the skin.
A biological electrode consists of an electrode element made of silver, silver/silver chloride, carbon and the like and a conductive composition connecting the electrode element to the skin and is applied to the surface of a living body in order to output some bioelectrical phenomena (e.g., electrocardiogram, electromyogram, etc.) or electrically stimulate the living body.
Normal human skin has an external layer called xe2x80x9chorny layerxe2x80x9d which protects the living body against the invasion of various foreign factors. When the skin is contacted with the dry atmosphere, the moisture is lost from the horny layer. Also, the moisture content in the horny layer is reduced as aging proceeds. In such cases, the electrical resistance of the horny layer is elevated. The surface of the skin is not smooth but uneven and has a complicated shape, for example, being curved.
When an electrode element is contacted directly with the skin surface containing less moisture in the horny layer, it is frequently observed that the contact of the electrode element to the skin is inhibited and thus the effective contact area is reduced. As a result, the contact resistance is elevated. In addition, the resistance of the horny layer per se has been elevated as described above. Thus the total electrical resistance is considerably elevated, which causes some troubles, for example, the bioelectrical signal thus output picks up noise, only an unstable record can be obtained, or no record can be obtained in some cases. When the surface of a living body is electrically stimulated via an electrode, the high resistance at the contact area brings about an increase in the current density and thus causes damages such as burn to the living body.
To solve these problems, conductive compositions in the form of liquid, jelly or gel are generally employed in biological electrodes so as to reduce the skin resistance between the living body and the electrode element. These conductive compositions contain a large amount of water and/or electrolytes such as NaCl or KCl which are externally absorbed by the skin horny layer to thereby reduce the skin resistance. Owing to the characteristics of the components, however, such a conductive composition per se has a low viscosity and a high flowability, which makes it difficult to stably sustain the conductive composition between the electrode element and the skin for a prolonged period of time. Thus the electrode element should be provided with a containment space or a holding means such as sponge for supporting the conductive composition. The electrode element should be further provided with an adhesive tape for fixing it on the skin surface. When an adhesive tape is used, however, repeated application and removal of the electrode element cause mechanical damage to the skin.
When an electrode element is applied to the skin surface for a long time via such a conductive composition in an ICU, Holter""s electrocardiography, etc., the conductive composition bears mechanical load due to body motion and external pressure. The conductive composition leaks from the electrode element to cause detachment of the electrode element or unstable contact of the electrode element with the skin, thus making it impossible to record the biological signals.
In addition, such a conductive composition is dried during application due to the evaporation of the moisture contained therein. Thus the skin resistance is elevated, thereby making the record of the biological signals unstable. Furthermore, the evaporation of the moisture contained in the conductive composition causes an increase in the chlorine ion concentration in the conductive composition, which induces skin irritation. After the removal of the electrode element, furthermore, such a conductive composition remains on the skin and causes rash.
There are electrode elements usable repeatedly and so-called disposable ones which are thrown away after being used once. In the former ones, a conductive composition is applied to the skin immediately before use. In the latter ones, on the other hand, a conductive composition has been preliminarily filled or incorporated into the electrode in many cases so that they can be easily applied. In the latter case, therefore, it is required to have a structure that the conductive composition is kept not dried until it is used and to store the conductive composition in an airtight package to thereby prevent it from drying during storage. Accordingly, an electrode element of the latter type should have a complicated structure as a whole with taking the use and storage thereof into consideration.
To solve the above-mentioned problems of the liquid conductive composition, there have been recently proposed solid gel conductive compositions which are not flowable but soft and sticky (cf. JP-B-62-44933, JP-B-63-43646, JP-B-63-59334 and JP-B-2-32892; the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined published Japanese patent applicationxe2x80x9d). Since stickiness can be imparted to a solid gel conductive composition per se, it can be advantageously employed in a biological electrode without using any adhesive tape for fixation which is essentially required in electrodes with the conventional liquid conductive compositions.
However, the largest disadvantage of these solid gel conductive compositions resides in that they cannot sufficiently reduce the impedance between the electrode and the skin.
The solid gel conductive composition functions to reduce the impedance of the horny layer by allowing the horny layer to absorb the moisture and electrolytes such as KCl or NaCl contained in the conductive composition. In general, moisture contained in a solid gel includes free water which can freely move and bonding water which cannot freely move. The free water largely contributes to the reduction of the impedance of the horny layer. Thus a conductive composition with a larger content of free water can achieve the better effect. However an increase in the free water content might result in the oozing of water onto the solid gel surface or loss of the stickiness, thus damaging the functions characteristic to solid gels.
Conventional solid gels frequently contain polyhydric alcohols such as glycerol and propylene glycol as a plasticizer. Such a plasticizer is nonionic and does not participate in conductivity, which elevates the electrical resistance of the conductive composition per se containing the plasticizer.
To lower the electrical resistance, electrolytes such as KCl and NaCl are added to the conductive composition. It is furthermore required to add water thereto so as to facilitate the smooth migration of these electrolytes. When the moisture content is increased so as to facilitate the smooth migration of the electrolytes, however, the composition suffers from a decrease in the stickiness and thus becomes unusable, as described above. In addition, the percutaneous absorption of excessive moisture or electrolytes causes skin disorders such as rash. Namely, there is the upper limit of the moisture content and thus the impedance of the conductive composition against the skin cannot be sufficiently lowered by this method.
An object of the present invention is to provide a conductive composition for a biological electrode capable of elevating the moisture content in the horny layer and lowering the impedance between the electrode and the skin while causing little irritation without depending on any increase in the moisture content or the amount of electrolytes.
Thus, the present invention provides a conductive gel composition for a biological electrode which functions to electrically and physically connect a living body to an electrode element and comprises at least the following components: (1) a radical-polymerizable unsaturated compound; (2) a moisturizer serving as a plasticizer and having a function of supplementing and promoting the physiological humidifying function of the skin horny layer; (3) a polymerization initiator; and (4) a crosslinking agent.
The present invention also provides a conductive gel composition for a biological electrode comprises a plasticizer selected from the group consisting of polyhydric alcohols, amines and ethers, as well as the above-described components (1)-(4).